A tungsten heavy alloy is composed of about 80% or more by weight of tungsten, and iron or copper, and especially where its tungsten content is more than about 90% by weight, the tungsten heavy alloy is called a tungsten superheavy alloy. Such a tungsten heavy alloy is becoming increasingly used in applications utilizing thermal expansion, such as thermal stress buffering for ceramic and metal materials, and applications requiring high mechanical strength, such as quills, shanks, and boring bars, as well as in such applications as automobile flyweights, spray nozzle weights, computer HDD weights, and VTR heads, which require a large weight though small in size.
Tungsten heavy alloys, including such tungsten superheavy alloys, have hitherto been produced by powder metallurgical techniques, because they contain a high melting-point tungsten. That is, W powder, Ni powder, and Fe powder or Cu powder are mixed in predetermined proportions, and the mixture powder is molded by a conventional press molding technique, such as pressing or CIP molding, the molded material being then sintered into a hard mass having a nearly perfect compact density. A similar powder metallurgical method is widely known for producing iron-base alloys.
However, such conventional powder metallurgical methods as mentioned above, wherein a molded material is obtained by press molding, have a disadvantage that the product to be produced is limited in configuration and dimensional accuracy. For example, press molding can produce no more than products of such a configuration as to permit the product to be monoaxially molded. CIP molding cannot provide high molding accuracy because molding is effected in a rubber mold, although it can produce a product of a tridimensional configuration. As such, in order to obtain the desired configuration for a final product, it is necessary to machine the product with respect to almost all portions thereof after the product has been sintered, which naturally means low productivity and increased costs.
When producing a composite product comprising a tungsten heavy alloy and an iron-base alloy or other metal material, it has been usual practice to join by silver brazing the alloy portions made to respective predetermined shapes by conventional powder metallurgical techniques, or to cast the tungsten heavy alloy portion, produced by a conventional powder metallurgical technique, in chills with an iron-base alloy or other metal material.
However, such a method does not provide a dependable junction or sufficient strength, and this constitutes a great limitation upon using the resulting product as a structural material.
In view of such disadvantages of the foregoing powder metallurgical methods, there have been developed methods as disclosed in Japanese Patent Publication No. 63-42682 and Japanese Patent Application Laid-Open Publication No. 62-250102, wherein metal or alloy powder is mixed with an organic binder and the mixture is injection-molded into a molded material which, in turn, is subjected to thermal decomposition in a non-oxidizing atmosphere or a similar debinding treatment for removal of the organic binder, the resulting product being then sintered.
Also, there has been known a method, as described in Japanese Patent Application Laid-Open Publication No. 62-249712, wherein a mixture of an organic binder and a material powder mass is injection-molded into a molded material which, in turn, is placed in a separate mold having a sufficient cavity, and wherein a mixture of same or different kind of material powder and an organic binder is injected into the cavity for being molded integrally with the previously molded material, the integral moldings being subjected to the step of debinding or binder removal and then sintered.
Various kinds of organic binders for use in mixture with the material powder have been known, including combinations of lubricants, such as atactic polypropylene, wax, and paraffin, with plasticizers, such as diethyl phthalate, as described in Japanese Patent Publication No. 51-29170; polyethylene, polystyrene, and beeswax, as described in Japanese Patent Application Laid-Open Publication No. 57-26105; and thermoplastic resins and silane or titanium coupling agents, as described in Japanese Patent Application Laid-Open Publication No. 55-113511.
A molded material produced by injection molding contains an organic binder and, therefore, must be heated for binder removal before it is sintered. In order to prevent the molded material from becoming deformed during that process, various methods have hitherto been in practice, including for example one in which the surface of the molded material is slightly oxidized for increasing the strength thereof, one in which such an amount of the binder as to permit the molded material to retain its form is intentionally retained, and one in which the binder removing step is carried out while the molded material is held as buried in a powdery alumina mass.
As separate means intended for this purpose, a debinding method utilizing an organic solvent has been proposed. In the specification of U.S. Pat. No. 4,765,950, for example, there is described a method wherein two kinds of organic binders, the one kind being soluble in a certain organic solvent, the other being sparingly soluble in the organic solvent, are used in combination, whereby the soluble organic binder will first be dissolved and extracted in the organic solvent so that open pores are formed in the molded material, the remaining sparingly soluble organic binder being then removed by heating.
In practice, however, in view of the fact that usually about 50% by volume of an organic binder is mixed with the material powder, it has been extremely difficult to inhibit the deformation of the molded product, even when the molded product is treated for binder removal prior to the sintering step, and further to completely remove the organic binder. In particular, such an injection molding method has been found to be impracticable for application to tungsten heavy alloys in its literal terms and also for application to other metals, for the following reasons.
First, when any existing method is employed in producing a tungsten heavy alloy product, the problem is that about 0.1% by weight of carbon will remain unremoved from the product after the step of debinding is carried out, with the result that the product is considerably degraded in strength and toughness by reason of the residual carbon. As such, the product thus produced is lower in strength and toughness than products made by a conventional powder metallurgical method using the pressure casting technique.
In order to obtain a product made of a tungsten heavy alloy material which meets both the strength and the toughness requirements of the product, it is essential that the residual carbon content be considerably lower than that in products made of any other metal material, such as an iron-base alloy. Additionally, it must be pointed out that such residual carbon is more likely to be present in a midinterior portion of the product, in the case where the product is relatively thick in section.
Second, in the binder removing stage, it has been usual practice to adopt such a low rate of temperature increase as not more than 20.degree. C./hr in order to prevent the occurrence of cracking and/or creep strain with respect to the product, considerable time being thus required for binder removal. This has been a new cause of low productivity.
Third, during the stage of binder removal from the injection molded product, whether by heating or by extraction with an organic solvent, the tungsten heavy alloy molded product is liable to deformation under its own weight because the specific gravity of the product is considerably large.
It may be conceivable to use a method such that the molded product is buried in a powdery alumina mass as has often been practiced for binder removing purposes, but it must be noted that such method has been developed in the art of producing products of ceramics and other metal materials, such as iron-base alloys, whose specific gravity is relatively small. Therefore, it is impracticable to completely prevent the deformation of the molded product if the method is applied as such to the tungsten heavy alloy.
Fourth, for the purpose of solvent extraction, it has been extremely difficult to find a suitable combination of two kinds of organic binders for use with tungsten heavy alloys which have good moldability and will not separate from each other, and which have different solubility characteristics relative to the organic solvent used for extraction. In the process of such extraction by dissolution with solvent, the fact that the specific gravity of the tungsten heavy alloy is relatively large has often been responsible for defects such as deformations and/or cracks caused to the surface and/or interior of the molded material.
Because of the foregoing problems, it has been difficult to obtain stable quality products on a mass production basis.
Fifth, since the molded material passed through the step of binder removal has a porosity of about 50%, it is necessary that the molded material be subjected to liquid phase sintering usually under maximum temperature conditions, that is, within a temperature range of from the melting point of nickel, iron or copper bond phase and up to +50.degree. C. thereabove, in order to bring the molded material to close proximity to the state of true density and, at same time, to facilitate the growth of tungsten particles to enable the molded material to have good toughness. In this case, when heating is effected continuously until the maximum temperature conditions are reached, the tungsten heavy alloy is likely to become deformed under its own weight because its bond phase tends to change abruptly into a liquid phase. Especially where products of a more complex configuration are required, the tungsten heavy alloy is liable to greater deformation; and as such it is impracticable to obtain a product having a high degree of dimensional accuracy.
Sixth, a problem exists with molded composites incorporating an iron-base alloy component formed integrally with a tungsten heavy alloy component. In Japanese Patent Application Laid-Open Publication No. 62-249712, for example, there is disclosed a method wherein a mixture of an organic binder and a certain metal powder material is injection-molded into a molded material which, in turn, is placed in a separate mold having a surplus cavity, and wherein a mixture of same or different kind of material powder and an organic binder is injected into the cavity for being molded integrally with the previously molded material, the integral moldings being subjected to the step of binder removal and then sintered.
However, most of the teachings given in such publication refer to cases in which same kinds of materials are used and, for the purpose of integrally complexing different kinds of materials into moldings and sintering the moldings, it is only stated therein that materials of a similar sintering temperature range should be selected, and that differences in their shrinkage behaviors due to sintering should be fully considered. In the case of a combination of such materials with a tungsten heavy alloy, it must be pointed out that sintering temperatures for the tungsten heavy alloy are generally 1300.degree.-1450.degree. C., while those for iron-base alloys are generally 1100.degree.-1300.degree. C. With such known method, therefore, as far as most tungsten heavy alloy compositions are concerned, it is impossible to sinter composite moldings of both tungsten heavy alloy and iron-base alloy components thereby to produce a tungsten heavy alloy--iron-base alloy composite product having high dimensional accuracy, a complex configuration, and yet having high strength and good toughness, in such a manner as to provide for high productivity.